Crude streams for the commercial production of olefins contain various compounds as impurities. Acetylenic and diene impurities need to be removed from the streams to produce acceptable quality olefin products. To produce olefins such as ethylene, propylene, butadiene, pentenes and the like, acetylenic impurities such as acetylene, methyl acetylene, vinyl acetylene, ethyl acetylene, 2-methyl-1-buten-3-yne and the like, as well as diene compounds, such as butadiene, propadiene, and the like, in various crude mixed C2-C5 streams need to be removed with minimum loss of useful materials such as ethylene, propylene, butenes, pentenes, and the like in the feed streams. The preferred technique for the purification in commercial practice is the selective hydrogenation of acetylenic and diene compounds over hydrogenation catalysts.
Crude C5 olefin-containing streams may include various dienes and acetylenes, which often must be removed before use of the C5 olefin-containing stream in downstream processing units, such s a downstream metathesis unit. In addition to the need to remove dienes and acetylenes, which produce coke and shorten metathesis catalyst run length, cyclopentene must also be removed from the C5 feed to a very low level, such as less than 0.5 wt. %, as cyclopentene may undergo undesirable ring-opening metathesis polymerization in the downstream metathesis unit. The very close boiling points of linear C5 olefins and cylcopentene, however, require that a very high reflux ratio be used during fractionation or catalytic distillation to sufficiently separate the cyclopentene, recovered in the bottoms fraction, from the linear pentenes, recovered in the overheads fraction.